Normal for diodes to be really hot?

[birchoak]

Howdy folks. Is it normal for a diode in, say, a power supply to be much hotter than all the components around it? For example, in an old Concept 2.0 receiver diode D5 in the power supply got up to about 230˚F when playing music at a healthy volume (but not pushed hard). Almost all of the components around it were more like 70 to 90˚F. I am in a cool (63˚) basement under florescent and LED lighting. The diode in question sits up off the board with insulated legs. I can provide a picture later if that helps but it's really a general question. I know a car engine should get hot, but what about diodes?

FYI the output transistors seem to stay pretty cool; 92˚ each I believe.

Thank you for indulging my curiosity.

 

[baconbadge]

A quick look on Mouser shows that modern diodes have ratings up to 150-200 degrees Celsius, which is over 300 degrees Fahrenheit. Really depends on what temp the diode is rated for. But if you're approaching the maximum temp rating, I'd replace 'em with new ones just to be safe. Also check all your voltages and compare to the schematic to make sure everything is on the up & up. 230˚F is approaching tube temperatures.

 

[birchoak]

A quick look on Mouser shows that modern diodes have ratings up to 150-200 degrees Celsius, which is over 300 degrees Fahrenheit. Really depends on what temp the diode is rated for. But if you're approaching the maximum temp rating, I'd replace 'em with new ones just to be safe. Also check all your voltages and compare to the schematic to make sure everything is on the up & up. 230˚F is approaching tube temperatures.

Yeah, it seemed kind of high to me. I will check voltages and when I can figure out what kind of diode it is I will replace it. Sadly, there is no SM for this receiver and the schematic doesn't include any voltages.

 

[baconbadge]

Try and post some pics. I'm sure someone can tell you what the correct replacement would be.

 

[roger2]

Normally I would say yes, it's normal for diodes in some roles to run hotter than other components. But 230F = 110C which is pretty hot. The fact that that diode is raised off the board means that the designer knew it would run hot. But is 230F too hot...? I don't know.

IMO replacing the diode might not solve a problem. If there is a problem, it could be that some other component(s) in that circuit was having issues. I would investigate there. It would be helpful to others reading this thread if you could post an excerpt of the schematic showing that diode and surrounding circuitry. Whatever you post please be sure it is readable. If posting directly to an AK post the image size limit is 800 pixels wide so a full schematic would not be readable.

If you do replace that diode, the parts list in the Service Manual should give you a part number. You could post that part number here to ask for substitutes if the original part is NLA

 

[audiodummy]

Usually rule of thumb is if it's too hot for your fingers (usually 60°C or so Tcase), it's too hot. Mostly because it's usually hotter at the actual semiconductor material (Tjunc), but this is just a general rule. Sometimes it's fine to run it hotter if the designer considers derating. However I'd say boiling water likely is too hot. However it's not the diode that's causing itself to be hot, it's the symptom of something else broken or not designed properly. Replacing it with an identical unit, the new diode likely will become hot as well.

You mention this is a PSU diode, perhaps an upgrade is needed. Is this a 1N4001 or what kind of diode at least - rectifier, zener, something else? Sometimes upgrading to an 1N5401 is sufficient to at least reduce temperatures as it can dissipate more heat. Undersized zener diodes will get quite hot too.

Yes the schematic (or even a picture) would be interesting to look so we at least know how it's being used, even if there's no service manual...

 

[birchoak]

Normally I would say yes, it's normal for diodes in some roles to run hotter than other components. But 230F = 110C which is pretty hot. The fact that that diode is raised off the board means that the designer knew it would run hot. But is 230F too hot...? I don't know.

IMO replacing the diode might not solve a problem. If there is a problem, it could be that some other component(s) in that circuit was having issues. I would investigate there. It would be helpful to others reading this thread if you could post an excerpt of the schematic showing that diode and surrounding circuitry. Whatever you post please be sure it is readable. If posting directly to an AK post the image size limit is 800 pixels wide so a full schematic would not be readable.

If you do replace that diode, the parts list in the Service Manual should give you a part number. You could post that part number here to ask for substitutes if the original part is NLA

Thank you for responding. There is no SM for this receiver; it is unobtanium. I also thought just replacing the diode might not be enough; why is so much heat being generated? I will take photos next time I'm at the bench and will find the schematic. Thank you again, kind sir.

 

[birchoak]

Usually rule of thumb is if it's too hot for your fingers (usually 60°C or so Tcase), it's too hot. Mostly because it's usually hotter at the actual semiconductor material (Tjunc), but this is just a general rule. Sometimes it's fine to run it hotter if the designer considers derating. However I'd say boiling water likely is too hot. However it's not the diode that's causing itself to be hot, it's the symptom of something else broken or not designed properly. Replacing it with an identical unit, the new diode likely will become hot as well.

You mention this is a PSU diode, perhaps an upgrade is needed. Is this a 1N4001 or what kind of diode at least - rectifier, zener, something else? Sometimes upgrading to an 1N5401 is sufficient to at least reduce temperatures as it can dissipate more heat. Undersized zener diodes will get quite hot too.

Yes the schematic (or even a picture) would be interesting to look so we at least know how it's being used, even if there's no service manual...

More great information. Thank you. I like the idea of upgrading to a beefier diode and also understand that it's not the diode itself that's the main problem.
I recapped the entire PS and power amp boards, so I don't think it's bad/leaking caps doing this. I will take some pictures for you guys and try to find the relevant part of the schematic (I am not very great reading schematics but this will be good practice).

 

[fred soop]

Is there a resistor someplace in series with the diode? If the voltage across the resistor could be measured, then current could be calculated.

 

[Pio1980]

Pictures would help.
If its a zener, it may run warm but shouldn't run hot. If its a rectifier, it may be defective with reverse leakage.

 

[gadget73]

or something else is wrong and causing excess current draw.

 

[Pio1980]

Still need to see what it is.

 

[birchoak]

Ok, here's a shot of the power supply. The unhappy diode is D5, and it is that black job with two white bands facing left. It is the third component directly under pin 12. I will take voltage readings at B+ and B- and figure out how to show you the right part of the schematic. But that's the poor little diode and boy, does he get hot! I know this is irrelevant, but the Concept 2.0 is one of the best-sounding receivers I have ever heard and it's underpinned with a build quality and layout second to none (in its class, anyways--it's not a Sansui Eight or an SX-1250, but it is a phenomenal instrument and worth every bit of the hassle).

 

[Pio1980]

I'm guessing it's a zener shunt regulator then. The end with the lower voltage to common should show it's operating voltage rating if it's ok.

 

[kfalls]

It doesn't appear to be the only component getting heat. The capacitors to the right (gold/black) also look to have gotten hot at one time. The diode does appear to be a higher current diode, so I wouldn't be surprised it's warm and the leads above the insulation appears a little scorched. It wouldn't cost that much to replace. You just need to determine if the problem is with the diode itself or with its associated components causing the diode to get hot.

 

[dondoucette]

I’d be replacing those electrolytics
The one just to the right looks like it spilled it’s guts
Electrolytics hate heat, that electrolytic might be a near dead short causing you some issues which may be contributing to the hotter than normal diode which in turn is destroying capacitor(s) etc etc

 

[RobRoy]

Heat = light. I have LED headlights on my car.

 

[gadget73]

Could be a zener, the symbol on the board would be extra confirmation. The zener diode symbol has little flags off either end of the bar.

The other clue is how its wired. Usually one end will have a resistor from some voltage source, the other end will be grounded. Could have that reversed, but it will sit between a voltage supply and ground (or a lower voltage supply) and a resistor will be involved to limit current.

 

[roger2]

You should do a close inspection of the back side of the board under those components. There could be compromised solder joints or PCB tracks/pads. If you do any soldering, be on the lookout for damaged or lifted pads.

 

[roger2]

Here is the PS section from the Concept 2.0 schematic in AK's database. BUT, I don't believe it matches the pic above. Can someone else please confirm or deny?

In the schematic it shows a bridge rectifier. The pic shows four individual rectifier diodes D503-D506. The schematic shows D505 as a zener. The pics shows D505 as one of the rectifiers. I see no D5 in this schematic.

@birchoak does your schematic look like the one posted below?